Microstructural banding and failure of a stainless steel
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3/4/04
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Microstructural Banding and Failure of a Stainless Steel A.C. STAUFFER, D.A. KOSS, and J.B. McKIRGAN The presence of microstructural bands in AL-6XN stainless steel plate has been examined. The bands, which consist of a high density of second-phase particles, are located near the midthickness of the plate, range in thickness up to 300 m, and are continuous over lengths up to 50 mm. Chemical analyses of the microstructural bands indicate elevated levels of chromium and molybdenum, while orientation imaging microscopy identified primarily sigma-phase particles within the bands; a small volume fraction of chi phase was also found. Tensile specimens oriented in the short transverse direction of the plate show low ductility and exhibit a large variation in failure strains, depending on the continuity of the bands as well as the presence of large precipitate particles within the bands. When oriented in either the longitudinal or the long transverse direction of the plate, circumferentially notched tensile specimens exhibit comparatively high ductility, although at high stress triaxialities, the material was susceptible to specimen splitting parallel to the tensile axis due to cracking along microstructural bands.
I. INTRODUCTION
SOLUTE segregation during casting of alloys is a common occurrence. For steels processed by casting and hot rolling, the segregation of substitutional alloying elements during the dendritic solidification often results in microstructural banding (References 1 through 6, for example). In low-alloy steels, the presence of significant amounts of manganese, chromium, and molybdenum causes solidification to occur over a large range of both temperatures and compositions with the result that the interdendritic regions have a high solute content. During subsequent rolling, these regions become elongated and can take the form of chemical or composition bands aligned parallel to the rolling direction. Upon cooling, the differences in solute chemistry between the high solute bands and the neighboring material then results in microstructural banding within the plate. Austenitic stainless steels are commonly known for their toughness and corrosion resistance. Of these steels, AL-6XN* *AL-6XN is a registered trademark of ATI Properties, Inc., Pittsburgh, PA.
is commonly referred to as a “super-austenitic” stainless steel with a high Ni content (24 wt pct Ni) and, importantly, it contains a significant amount of Mo (6 wt pct) in addition to the level of Cr (20-21 wt pct) typical of austenitic grades.[7] Given certain thermal histories, some stainless steels may be susceptible to the formation of potentially detrimental intermetallic phases, such as the sigma phase.[7–13] A key to the formation of sigma is the presence of high levels of Cr and Mo.[14,15] In contrast, elements such as carbon, nickel, and nitrogen retard its formation,[7,10,14,] and significantly, AL-6XN has a relatively high nitrogen content (0.2 wt pct). The relatively high levels of Cr and Mo in the AL-6XN alloy
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